For the production of silane processes are known which are based on metallurgic silicon or else halosilanes as the starting material.
Of the halosilanes,SiF.sub.4 is of particular technical interest due to its availability and its low cost price. It accumulates in large quantities in the exhaust gas during wet extraction of raw phosphates and can be obtained therefrom with a high degree of purity according to various processes, some of which have been tried out on an industrial scale, and described, for example, in Proc. Fert. Soc. (PFRSAZ) V 163,23 pp, 1977. In the form of a gaseous compound, SiF.sub.4 can be easily purified further, if desired. With its 27 wt. % silicon content, SiF.sub.4 has the highest silicon content of all silicon perhalides, and a higher silicon content than trochlorosilane, as well. It is therefore a very economical starting material for producing other silicon compounds.
German Pat. No. 1 034 159 discloses the conversion of silicon halides with complex alkali hydride boron trialkyl compounds or alkali hydride aluminum alcoholate compounds, dissolved in an organic solvent, into silicon hydrides. The disadvantages are that the resulting silicon hydride is contaminated by solvent vapors and boron or aluminum compounds, and the process can only be carried out discontinuously.
It is also known to convert SiF.sub.4 with calcium hydride into silane at temperatures between 250.degree. and 300.degree. C. (Paul Hagenmueller, Robert de Pape, Comp. rend. 251, 2032-4 (1960); Robert de Pape, Ann. Chim., t. 8, 1963, p. 185-195). The disadvantage in this case is that the reaction soon comes to a standstill and silicon is precipitated. Thus, only 12% conversion was ascertained, for example, after a reaction time of 24 hours at 290.degree. C.
It is disclosed in U.S. Pat. No. 2,933,374 to convert SiF.sub.4 with hydrogen in an electric arc into fluoric silanes which are disproportionated via sodium fluoride into silane at temperatures between 125.degree. and 300.degree. C. The disadvantage in this case is that the conversion of SiF.sub.4 into fluoric silanes has only a small yield in spite of a high use of energy, and SiF.sub.4 is lost during the disproportionation with sodium fluoride due to the formation of sodium hexafluosilicate.
German Pat. No. 1 080 077 discloses a process and an apparatus for hydrating halogen-substituted compounds of the elements boron, silicon or germanium by reaction with alkali or alkaline earth hydrides in a melt of an alkali, ammonium or alkaline earth halide or halide mixture. However, this process requires the use of metallic hydrides which are stable in the melt at the relatively high working temperatures, e.g. lithium hydride. On the other hand, no information is stated in the description of either the process or the apparatus on the relation between the working temperature in the melt and the thermal decomposition of the metallic hydrides. On the contrary, it is assumed that the metallic hydrides are stable in the melt at the relatively high working temperatures.
However, if it is taken into account that sodium hydride and potassium hydride, for example, already begin to decompose at approx. 300.degree. C. and that magnesium hydride already decomposes completely at approx. 280.degree. C. (S. Ullmann, 4th edition, Vol. 13, p. 114 and 116, or E. Wiberg and E. Amberger, "Hydrides of the Elements of Main Groups I-IV", Elsevier 1971, p. 29 and 35), the use of precisely these metallic hydrides which are readily accessible technically is ruled out. Magnesium hydride, that alkaline earth hydride which may be manufactured of magnesium, the only inexpensive alkaline earth metal which is available on an industrial scale, cannot be used for the process in the publication because of thermal decomposition. In order to reduce the high cost prices for the metallic hydrides to be used, it is further proposed to design the reactor so that it can be used not only for silane synthesis, but also simultaneously for obtaining alkali or alkaline earth metals electrolytically and subsequently synthesizing the corresponding metallic hydride. Reactors suitable for all these reactions are of course elaborate and susceptible to disturbance.